1. Field of the Invention
The present invention relates to a radiographic image data correction method and apparatus that performs, based on offset correction data, offset correction on radiographic image data that have been read out from a radiation image detector. Further, the present invention relates to a radiography apparatus.
2. Description of the Related Art
Conventionally, various kinds of radiation image detectors that record radiographic images of a subject by irradiation with radiation that has been transmitted through the subject have been proposed and used in medical fields and the like.
The radiation image detectors are, for example, a radiation image detector using amorphous selenium that generates charges by irradiation with radiation. As such a radiation image detector, a light-readout-type radiation image detector has been proposed.
For example, as the light-readout-type radiation image detector, a radiation image detector including a first electrode layer, a photoconductive layer for recording, a charge transfer layer, a photoconductive layer for readout, and a second electrode layer has been proposed. In the radiation image detector, these layers are superposed one on another in the mentioned order. The first electrode layer transmits radiation that carries a radiographic image, and the photoconductive layer for recording generates charges by irradiation with the radiation that has been transmitted through the first electrode layer. The charge transfer layer acts as an insulator with respect to charges of one of the polarities generated in the photoconductive layer for recording, and acts as a conductor with respect to charges of the other polarity generated in the photoconductive layer for recording. The photoconductive layer for readout generates charges by irradiation with readout light, and the second electrode layer includes transparent linear electrodes that transmit the readout light and light-shield linear electrodes that block the readout light.
When a radiographic image is recorded in the light-readout-type radiation image detector, as described above, first, a negative voltage is applied to the first electrode layer of the radiation image detector by a high-voltage source. While the negative voltage is applied to the first electrode layer, radiation that has been transmitted through the subject, and which carries a radiographic image of the subject, is output to the radiation image detector from the first electrode layer side of the radiation image detector.
Further, the radiation that has been output to the radiation image detector is transmitted through the first electrode layer, and irradiates the photoconductive layer for recording. Further, pairs of charges (dipoles) are generated in the photoconductive layer for recording by irradiation with the radiation. The positive charges of the dipoles are combined with negative charges charged in the first electrode layer, and disappear. Further, the negative charges of the dipoles are stored, as latent image charges, in a charge-storage portion that is formed at the interface between the photoconductive layer for recording and the charge transfer layer.
Next, readout light is output to the radiation image detector from the second electrode layer side while the first electrode layer is grounded. The readout light is transmitted through the transparent linear electrodes, and irradiates the photoconductive layer for readout. Positive charges are generated in the photoconductive layer for readout by irradiation with the readout light, and the positive charges are combined with the latent image charges in the charge-storage portion. Further, an electric current flows when the negative charges generated in the photoconductive layer for readout combine with the positive charges charged in the transparent linear electrodes and the light-shield linear electrodes, and the electric current is detected by a charge amplifier connected to the light-shield linear electrodes. Accordingly, radiographic image data corresponding to the radiographic image are read out.
Here, performing so-called offset correction on the radiographic image data that are output from the radiation image detector has been proposed.
Offset correction data that are used for the offset correction are obtained by applying a high voltage to the first electrode layer, and by performing readout while the radiation image detector is not irradiated with radiation. Since the offset correction data change according to the condition or state of the radiation image detector, it is desirable that the offset correction data are obtained at predetermined intervals.
However, since offset correction data need to be generated based on offset image data representing a plurality of images to perform accurate offset correction, long time is required to obtain the offset image data for the plurality of images. Therefore, the operation efficiency becomes lower.
Further, in recording of the radiographic image as described above, the time period of irradiation of radiation is controlled based on the radiography condition, such as the region of the body to be imaged. Further, the time period of applying the high voltage is controlled in various manners. Since offset generated in the radiation image detector differs depending on the time period of applying the high voltage, offset correction data for each time period of applying the high voltage are necessary to perform highly accurate offset correction. Therefore, it is necessary to obtain offset image data representing a plurality of images for each time period of applying the high voltage. Hence, longer time becomes necessary for processing.
Japanese Unexamined Patent Publication No. 2006-305228 proposes a method for increasing the length of intervals of updating correction data by determining the update timing of the correction data by monitoring the image data output from the radiation image detector. However, it is impossible to reduce the time period of update processing, itself.
Further, in addition to the offset correction, sensitivity correction is performed on the radiographic image data output from the radiation image detector. It is desirable that the sensitivity correction data that are used for sensitivity correction are updated at predetermined intervals in a manner similar to the offset correction data. Further, there is a problem that a very long time period is required to obtain the sensitivity correction data in a manner similar to the obtainment of the offset correction data.